Getting selected text in an iframe plugin

Hello.

I have a iframe plugin that generates a link from a cms content list. I want to add the link around the selected text when the ok button is clicked. It all works, except i can't get the selected text.

I have tried using the answer supplied here:

http://stackoverflow.com/questions/2385609/getting-selected-text-win-ckeditor-plugin

so like this:

var okListener = function(ev) {
      console.log(CKEDITOR);
      var mySelection = CKEDITOR.editor.getSelection();
      if (CKEDITOR.env.ie) {
          mySelection.unlock(true);
          selectedText = mySelection.getNative().createRange().text;
      } else {
          selectedText = mySelection.getNative();
      }      
      console.log(selectedText);

But it dosn't return the text. I get error:

"37Uncaught TypeError: Object function (b,c,d,e){var f=this;f._={instanceConfig:b,element:c,data:e};f.elementMode=d||0;a.event.call(f);f._init();} has no method 'getSelection'"


I'm using the latest version of ckeditor and im creating my plugin like this:

CKEDITOR.plugins.add('tree',{
    requires: ['iframedialog'],
    init:function(editor){
        CKEDITOR.dialog.addIframe('tree_dialog', 'Content Tree', this.path + '../../../admin/contents/editor',650,400,function(){/*oniframeload*/})
        var cmd = editor.addCommand('tree', {exec:tree_onclick})
        cmd.modes={wysiwyg:1,source:1}
        cmd.canUndo=false
        editor.ui.addButton('tree',{ label:'Go ahead, pick a page...', command:'tree', icon:this.path+'tree.png' })
    }
})

function tree_onclick(editor)
{
    editor.openDialog('tree_dialog');            
}

Any suggestions how to get the selected text would be very much appreciated,

Thanks

will